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Date:   Mon, 8 Feb 2021 07:35:50 +0200
From:   Eli Cohen <elic@nvidia.com>
To:     Si-Wei Liu <si-wei.liu@oracle.com>
CC:     <mst@redhat.com>, <jasowang@redhat.com>,
        <linux-kernel@vger.kernel.org>,
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Subject: Re: [PATCH 1/3] mlx5_vdpa: should exclude header length and fcs from
 mtu
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On Sat, Feb 06, 2021 at 04:29:22AM -0800, Si-Wei Liu wrote:
> When feature VIRTIO_NET_F_MTU is negotiated on mlx5_vdpa,
> 22 extra bytes worth of MTU length is shown in guest.
> This is because the mlx5_query_port_max_mtu API returns
> the "hardware" MTU value, which does not just contain the
> Ethernet payload, but includes extra lengths starting
> from the Ethernet header up to the FCS altogether.
> 
> Fix the MTU so packets won't get dropped silently.
> 
> Signed-off-by: Si-Wei Liu <si-wei.liu@oracle.com>

Acked-by: Eli Cohen <elic@nvidia.com>

> ---
>  drivers/vdpa/mlx5/core/mlx5_vdpa.h |  4 ++++
>  drivers/vdpa/mlx5/net/mlx5_vnet.c  | 15 ++++++++++++++-
>  2 files changed, 18 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/vdpa/mlx5/core/mlx5_vdpa.h b/drivers/vdpa/mlx5/core/mlx5_vdpa.h
> index 08f742f..b6cc53b 100644
> --- a/drivers/vdpa/mlx5/core/mlx5_vdpa.h
> +++ b/drivers/vdpa/mlx5/core/mlx5_vdpa.h
> @@ -4,9 +4,13 @@
>  #ifndef __MLX5_VDPA_H__
>  #define __MLX5_VDPA_H__
>  
> +#include <linux/etherdevice.h>
> +#include <linux/if_vlan.h>
>  #include <linux/vdpa.h>
>  #include <linux/mlx5/driver.h>
>  
> +#define MLX5V_ETH_HARD_MTU (ETH_HLEN + VLAN_HLEN + ETH_FCS_LEN)
> +
>  struct mlx5_vdpa_direct_mr {
>  	u64 start;
>  	u64 end;
> diff --git a/drivers/vdpa/mlx5/net/mlx5_vnet.c b/drivers/vdpa/mlx5/net/mlx5_vnet.c
> index dc88559..b8416c4 100644
> --- a/drivers/vdpa/mlx5/net/mlx5_vnet.c
> +++ b/drivers/vdpa/mlx5/net/mlx5_vnet.c
> @@ -1907,6 +1907,19 @@ static int mlx5_get_vq_irq(struct vdpa_device *vdv, u16 idx)
>  	.free = mlx5_vdpa_free,
>  };
>  
> +static int query_mtu(struct mlx5_core_dev *mdev, u16 *mtu)
> +{
> +	u16 hw_mtu;
> +	int err;
> +
> +	err = mlx5_query_nic_vport_mtu(mdev, &hw_mtu);
> +	if (err)
> +		return err;
> +
> +	*mtu = hw_mtu - MLX5V_ETH_HARD_MTU;
> +	return 0;
> +}
> +
>  static int alloc_resources(struct mlx5_vdpa_net *ndev)
>  {
>  	struct mlx5_vdpa_net_resources *res = &ndev->res;
> @@ -1992,7 +2005,7 @@ static int mlx5v_probe(struct auxiliary_device *adev,
>  	init_mvqs(ndev);
>  	mutex_init(&ndev->reslock);
>  	config = &ndev->config;
> -	err = mlx5_query_nic_vport_mtu(mdev, &ndev->mtu);
> +	err = query_mtu(mdev, &ndev->mtu);
>  	if (err)
>  		goto err_mtu;
>  
> -- 
> 1.8.3.1
>